refactor(change): do API changes to buffer without curbuf switch

Most of the messy things when changing a non-current buffer is
not about the buffer, it is about windows. In particular, it is about
`curwin`.

When editing a non-current buffer which is displayed in some other
window in the current tabpage, one such window will be "borrowed" as the
curwin. But this means if two or more non-current windows displayed the buffers,
one of them will be treated differenty. this is not desirable.

In particular, with nvim_buf_set_text, cursor _column_ position was only
corrected for one single window. Two new tests are added: the test
with just one non-current window passes, but the one with two didn't.

Two corresponding such tests were also added for nvim_buf_set_lines.
This already worked correctly on master, but make sure this is
well-tested for future refactors.

Also, nvim_create_buf no longer invokes autocmds just because you happened
to use `scratch=true`. No option value was changed, therefore OptionSet
must not be fired.

1 files changed, 17 insertions, 5 deletions

diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 1451a8404c..b100942594 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -348,9 +348,8 @@ int ml_open(buf_T *buf)
   }
 
   // Fill in root pointer block and write page 1.
-  if ((hp = ml_new_ptr(mfp)) == NULL) {
-    goto error;
-  }
+  hp = ml_new_ptr(mfp);
+  assert(hp != NULL);
   if (hp->bh_bnum != 1) {
     iemsg(_("E298: Didn't get block nr 1?"));
     goto error;
@@ -1170,7 +1169,7 @@ void ml_recover(bool checkext)
     // Recovering an empty file results in two lines and the first line is
     // empty.  Don't set the modified flag then.
     if (!(curbuf->b_ml.ml_line_count == 2 && *ml_get(1) == NUL)) {
-      changed_internal();
+      changed_internal(curbuf);
       buf_inc_changedtick(curbuf);
     }
   } else {
@@ -1180,7 +1179,7 @@ void ml_recover(bool checkext)
       int i = strcmp(p, ml_get(idx + lnum));
       xfree(p);
       if (i != 0) {
-        changed_internal();
+        changed_internal(curbuf);
         buf_inc_changedtick(curbuf);
         break;
       }
@@ -2495,6 +2494,19 @@ int ml_delete(linenr_T lnum, bool message)
   return ml_delete_int(curbuf, lnum, message);
 }
 
+/// Delete line `lnum` in buffer
+///
+/// @note The caller of this function should probably also call changed_lines() after this.
+///
+/// @param message  Show "--No lines in buffer--" message.
+///
+/// @return  FAIL for failure, OK otherwise
+int ml_delete_buf(buf_T *buf, linenr_T lnum, bool message)
+{
+  ml_flush_line(buf);
+  return ml_delete_int(buf, lnum, message);
+}
+
 static int ml_delete_int(buf_T *buf, linenr_T lnum, bool message)
 {
   if (lnum < 1 || lnum > buf->b_ml.ml_line_count) {